In Neurology, Clinical Neuroscience, and Cognitive Neuroscience there is considerable interest in mapping brain activity with millisecond temporal resolution using MEG (Magnetoencephalography). Current source activity in the brain generates magnetic fields that are detected by Superconducting Quantum Interference Device (SQUID) coils maintained at near absolute-zero temperature. SQUID coils have non-ideal sensitivity in the frequency range of MEG (< 100 Hz). Cryogenic operation requires placing the SQUID coils at a distance of 2-3 cm above the scalp, reducing spatial resolution. SQUID MEG coils are expensive to construct and maintain, which has led to the deployment of MEG systems with poor spatial resolution due to the limited sampling of the magnetic field at a significant distance above the scalp. The tremendous potential of MEG for localization of brain activity (known as Magnetic Source Imaging), has not been realized due to these limitations and the advantages of MEG over electroencephalography (EEG) for localization of brain function remains to be widely exploited by researchers or clinicians. In this project, we propose a new method to measure the magnetic fields at the scalp at room temperature using an atomic-optical technique to measure magnetic fields that overcomes all of the limitations of current MEG technology. We will use a unique fiber- optic Sagnac interferometer, which was originally developed for physics applications, as the detection scheme for magnetometers and gradiometers that operate at room temperature and can be placed against the scalp. The resulting Sagnac MEG technology has higher sensitivity (at the quantum noise limit of 0.01 femtoTesla), better spatial resolution, and dramatically lower cost than SQUID MEG. Tri-axis gradiometers that record all 3 gradients of the brain's magnetic field will be constructed and tested with phantom heads and experiments in human subjects. A novel spatial-temporal encoding and decoding scheme will be developed for simultaneous recording from multiple Sagnac gradiometers. Simulations, phantom experiments, and human experiments will be carried out to inform the design of future whole-head Sagnac MEG systems.